Spectral Induced Polarization imaging to monitor seasonal and annual dynamics of frozen ground at a mountain permafrost site in the Italian Alps

Abstract

Abstract. We investigate the application of spectral induced polarization (SIP) monitoring to understand seasonal and annual variations of freezing and thawing processes in permafrost, in particular with regard to the frequency-dependence of the subsurface electrical properties. We installed a permanent SIP monitoring profile at a high mountain permafrost site in the Italian Alps in 2019 and collected SIP data in the frequency range between 0.1–75 Hz over 3 years. Complementary seismic data were acquired, which, together with borehole data, were used to aid interpretation of the SIP imaging results. In particular, we investigated the phase frequency effect (ϕFE), i.e., the change of resistivity phase with frequency. We observe that this parameter (ϕFE) is strongly sensitive to temperature changes and might be used as a proxy to delineate spatial and temporal changes of ice content in the subsurface, providing information not accessible through electrical resistivity tomography (ERT) or single-frequency IP measurements. Temporal changes in ϕFE are validated through laboratory SIP measurements on samples from the site in controlled freeze-thaw experiments. We demonstrate that SIP is capable of resolving temporal changes in the thermal state and the ice/water ratio associated with seasonal freeze-thaw processes. We investigate the consistency between the ϕFE observed in field data and ground water and ice content estimates derived from petrophysical modelling of ERT and seismic data.